The present invention relates to a radio wave absorbent for use in an anechoic chamber, a radio wave absorptive wall and the like, which is composed of a nickel-zinc system ferrite.
Recently, with the progress of information communication technique or the prevalence of various electric apparatus, the influence of unnecessary electromagnetic noises exerted onto precision apparatus associated devices has posed problems. For the measurement of electromagnetic noises, an anechoic chamber where there is no reflection of electromagnetic waves is used, and a radio wave absorbent is used in the inner wall of the anechoic chamber. Moreover, in order to prevent a reception trouble caused by the reflection of television waves by high-rise buildings and the like, the radio wave absorbent is used in the outer wall of a building and the like. In addition, a large amount of such a radio wave absorbent is used in the inner wall of an anechoic chamber, the outer wall of a building and the like. Therefore, there has been a demand for the reduction of the production cost for the radio wave absorbent.
As a conventional radio wave absorbent, for example, use is made of a radio wave absorbent having characteristics such that a reflectivity in a frequency band of 40 MHz to 450 MHz is -20 dB or less. As such a radio wave absorbent, for example, a radio wave absorbent obtained by sintering a magnesium-zinc system ferrite material (Japanese Patent Application Laid-open Specification Nos. 72925/1989 and 301524/1989, and the like), and a radio wave absorbent obtained by sintering a nickel-zinc system ferrite material (Japanese Patent No. 2794293, Japanese Patent Application Laid-open Specification Nos. 129123/1993, 200303/1991 and 84622/1994, and the like) are exemplified.
The cost for a raw material used for producing a magnesium-zinc system ferrite material is relatively low. However, this ferrite material needs to be sintered at a temperature as high as 1,250.degree. C. or more, and hence, there has been a problem in that a high-temperature sintering furnace especial for this magnesium-zinc system ferrite material is required. Further, the matching thickness of the radio wave absorbent obtained by sintering the magnesium-zinc system ferrite material is as large as about 8 mm. Therefore, when such a radio wave absorbent is used in the inner wall of an anechoic chamber, the outer wall of a building and the like, the reduction of the total weight of the radio wave absorbent used is inevitably limited.
On the other hand, each of Japanese Patent No. 2794293 and Japanese Patent Application Laid-open Specification No. 129123/1993 discloses a radio wave absorbent comprising a nickel-zinc system ferrite material in which the range of the composition for nickel oxide, copper oxide, zinc oxide and iron oxide is defined. In this connection, it should be noted that, particularly, with respect to the anechoic chamber where an electromagnetic noise of a precision apparatus associated device is measured, a frequency band for the evaluation of the electromagnetic noise is standardized, that is, it is necessary that the reflectivity in a frequency band of 30 to 1,000 MHz be -20 dB or less. However, in the nickel-zinc system ferrite material disclosed in each of Japanese Patent No. 2794293 and Japanese Patent Application Laid-open Specification No. 129123/1993, the range of the composition which satisfies characteristics such that the matching thickness is 6.5 mm or less and the reflectivity at 30 MHz or more is -20 dB or less is very narrow. Therefore, it was difficult to control the production of such a radio wave absorbent.
In addition, Japanese Patent Application Laid-open Specification No. 200303/1991 discloses a radio wave absorbent comprising a nickel-copper-zinc ferrite which contains 7% by weight or less of titanium oxide. Further, Japanese Patent Application Laid-open Specification No. 84622/1994 discloses a radio wave absorbent comprising a nickel-copper-zinc ferrite which contains at least one subcomponent selected from 0.05% by weight or less of silicon dioxide and 0.10% by weight or less of manganese monoxide, wherein titanium dioxide, vanadium pentaoxide and hafnium oxide are contained as additives. There is a report that the addition of this titanium dioxide is effective for lowering the lower limit of the frequency band satisfying the reflectivity of -20 dB or less.
However, generally, when titanium dioxide is added to the nickel-zinc system ferrite as well as the magnesium-zinc system ferrite and the manganese-zinc system ferrite, a solid solution occurs not only at the inside of a crystal but also in a grain boundary phase. Therefore, when titanium dioxide, vanadium pentaoxide, hafnium oxide and the like in high concentrations constitute a grain boundary phase having no magnetic property as mentioned above, the matching thickness is caused to become large. The matching thickness exerts a remarkable influence on the total weight of the radio wave absorbent used in the inner wall of an anechoic chamber or the outer wall of a building or the like, and hence, the reduction of the matching thickness is always desired for the radio wave absorbent.